1. Field of the Invention
The present invention relates to a variable gain amplifier circuit. In particular, the invention relates to a variable gain amplifier circuit and a method of correcting a DC offset of the variable gain amplifier circuit for correcting a DC offset involved in an output signal of an amplifier circuit.
2. Description of Related Art
A differential amplifier circuit composed of MOS transistors has been known to have a DC offset in an output voltage due to a threshold voltage mismatch between MOS transistors as a differential pair which results from a variation in gate width between the MOS transistors, a mismatch between load resistors as components of the differential amplifier circuit, or the like.
A method of correcting a DC offset involved in the output voltage of such a differential amplifier circuit has been hitherto known. For example, Japanese Unexamined Patent Application Publication No. 9-64666 discloses a variable gain amplifier circuit including a differential amplifier circuit having a variable gain function and correcting a DC offset of an output voltage.
The variable gain amplifier circuit as disclosed in Japanese Unexamined Patent Application Publication No. 9-64666 is configured so as to switch between an offset detection mode and a signal amplifying mode, and an offset storage unit stores a DC offset amount detected in the offset detection mode. Here, the term “offset storage unit” means a capacitor for storing and outputting a DC offset amount as an analog value, and a memory for storing a digital value corresponding to a DC offset correction voltage output by a D/A converter (DAC). Further, in the signal amplifying mode, DC offset correction is executed such that a signal voltage from which an output voltage corresponding to a DC offset amount stored in an offset storage unit is subtracted is applied to a signal amplifying unit.
However, the DC offset amount varies depending on a gain of a signal amplifying unit. Hence, the conventional variable gain amplifier circuit as disclosed in Japanese Unexamined Patent Application Publication No. 9-64666 faces a problem in that a DC offset amount should be stored in the offset storage unit again at the time of changing the gain. Referring to FIG. 14, this problem is discussed.
FIG. 14 shows a configuration of the conventional variable gain amplifier circuit 8. An amplifying unit 81 is a differential amplifier circuit having a variable gain function. A gain switching unit 82 switches a gain of the amplifying unit 81 in accordance with a gain switching signal. Further, an input offset correction unit 83 is a circuit for subtracting an input offset correction voltage VOC—in as described below from an input signal voltage of the amplifying unit 81 to prevent the DC offset.
The DC offset VOS includes a fixed offset component VOS—fix that is fixed independently of a gain change due to a variation of a load resistor or current source of the amplifying unit 81 and an input offset component VOS—in that depends on a variation on the input side such as a variation in threshold voltage of the differential pair and changes in proportion to a gain. That is, the voltage VOS can be represented by Expression (1) below as a function of a gain A of the amplifying unit 81.VOS(A)=VOS—in*A+VOS—fix  (1)
The variable gain amplifier circuit 8 executes DC offset correction by subtracting the input offset correction voltage VOC—in from the input signal voltage of the amplifying unit 81 such that the voltage VOS derived from Expression (1) equals 0. That is, a correction voltage VOC—in is determined to establish Expression (2) below:VOS(A)=(VOS—in−VOC—in)*A+VOS—fix=0  (2)
Based on Expression (2) above, the voltage VOC—in that makes the voltage VOS zero can be represented by Expression (3) below. The correction voltage VOC—in represented by Expression (3) is stored in the input offset correction unit 83, and the generation of the DC offset VOS can be suppressed by subtracting the calculated voltage VOC—in from the input signal voltage of the amplifying unit 81.VOC—in=VOS—fix/A+VOS—in  (3)
As is apparent from Expression (3), however, the input offset correction voltage VOC—in depends on the gain A of the amplifying unit 81. Thus, at the time of changing the gain A, a value of the input offset correction voltage VOC—in for correcting the offset should be changed. Accordingly, the conventional variable gain amplifier circuit 8 needs to store the correction voltage again in the input offset correction unit 83 at the time of changing the gain of the amplifying unit 81.
For example, if the input offset correction unit 83 stores the correction voltage VOC—in in the capacitor, it is necessary to take enough time to charge the capacitor up to a voltage corresponding to the DC offset amount once again. In addition, if the input offset correction unit 83 is the DAC, it is necessary to enough time to determine a DC offset amount based on an output signal of the amplifying unit 81, enough time to control the DAC in accordance with the determined DC offset amount, or enough time to read a correction value previously recorded as a correction amount for each gain from a memory and set the value to the DAC once again.
As discussed above, the conventional variable gain amplifier circuit should execute DC offset correction each time a gain is changed, and time necessary for changing the gain is limited by offset correction time. This causes a problem in that the gain cannot be changed at high speeds.
For example, a receiver of a radio communication apparatus conforming to IEEE802.11a or other such wireless LAN standards needs to change a gain and adjusts a DC offset within a limited preamble period to amplify a reception signal after the preamble period. As a result, there is a possibility that if it takes much time to change a gain, a preparation for reception is not completed during the preamble period.
Incidentally, in some structures, a capacitor for cutting a DC component is inserted on the output side of the variable gain amplifier circuit to thereby cut DC components of an output voltage. In this case as well, it takes much time to converge a DC offset to zero while charging a capacitor. Thus, such structures are not appropriate for a variable gain amplifier circuit that needs to switch a gain at high speeds.
As mentioned above, the conventional variable gain amplifier circuit that needs to recorrect a DC offset each time a gain of the variable gain amplifier circuit is changed has a problem in that it takes much time to recorrect a DC offset and thus a gain cannot be changed at high speeds.